Conventionally, as means for identifying the authenticity of such articles as information storage cards including magnetic cards, stocks and bonds, tickets and commercial goods, a system of affixing a hologram marking, which is difficult to forge, on the article to be identified as an identification seal was proposed, for instance, in Japanese utility model laid-open publication No. 61-182580. As such identification seals are intended to be visually identified, they are normally affixed to the parts of articles to be identified which are easily noticeable from outside. Therefore, because designing of the external appearance of an article must take into account the position, shape and color of the identification seal that is going to be affixed thereto, freedom of design can be severely hampered, and in some cases the identification seal may destroy the balance in the design to such an extent that the appearance of the article may be severely impaired.
In Japanese patent laid-open publication No. 3-71383 filed by the applicant of this application, it was proposed to provide, on the surface of an article, a hologram region serving as an identification seal having a diffractive property unique to the article. In Japanese patent application No. 3-259636 (not yet published) filed by the same applicant and directed to an invention using the above-mentioned structure, an identification region made of a material which reflects, with a unique diffractive property, only the light having a prescribed wavelength, for instance light in the infrared band, which is not in the range of visible light, is affixed to such articles as information storage cards, stocks and bonds and tickets so that the identification region is normally transparent, but can be optically identified by using detecting light having the prescribed wavelength. According to this structure, the authenticity of the article can be identified with high accuracy, the forgery of the identification region is made more difficult, and the freedom in the design of the article is not in any way hampered.
However, in reality, it is difficult to obtain a layer which reflects only the light of a certain wavelength to a high degree, and the light to be reflected may well partly penetrate the layer and may be reflected by the surface of the article itself. In particular, when the surface of the article is metallic, and has a high reflective coefficient, the light reflected by the surface of the article may reach the light receiving region of the identification system along with the light reflected by the identification region whereby an erroneous detection may be resulted, and an authentic article may not be identified as such.